Semiconductor device using bumps, method for fabricating same, and method for forming bumps

ABSTRACT

A semiconductor device able to maintain a bonding state between a bump and an electrode and having high reliability even under thermal stress, wherein a sealing resin is interposed to bond the electrodes and bumps between a wiring board formed with a plurality of electrodes and an IC chip formed with a plurality of bumps, the bumps being formed under the condition that the following formula is satisfied. 
     
       
         100&lt;(( ΦA×F )/ H )&lt;125 
       
     
     where ΦA represents the top diameter of a bump bonded with an electrode, H the height of a bump projecting from the IC chip and bonded with an electrode, and F the linear thermal expansion coefficient of the sealing resin.

RELATED APPLICATION DATA

The present application is a divisional of U.S. application Ser. No.09/844,874 filed Apr. 27, 2001 now U.S. Pat. No. 6,614,111, which claimspriority to Japanese application P2000-134327 filed Apr. 28, 2000. Thepresent application claims priority to each of these previously filedapplications.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device having bumps, amethod for fabricating the same, and a method for forming bumps.

More specifically, the present invention relates to a semiconductordevice having bumps able to ease thermal stress to prevent damage due tothermal stress and therefore of high reliability, a method forfabricating the same, and a method for forming bumps.

Along with the increasingly small size of electronic apparatuses,attempts have been made to use compact semiconductor packages of sizessimilar to the size of a chip of a flip-chip structure. A large numberof electronic circuits are integrated on such a small package, so manyconnection terminals are necessary. On the other hand, due to thereduced size, the problem arises that the space for arranging theseconnection terminals is insufficient. In such a small semiconductorpackage, DIPs or other connection terminals of the related art cannot beused.

As a solution to this problem, attempts have been made for flip chipmounting where a large number of small projecting electrodes (bumps) areformed on the bottom surface of a semiconductor integrated circuit chip,many electrodes are formed on a printed wiring board at positionscorresponding to those bumps, and the electrodes on the wiring board andthe bumps formed on the semiconductor integrated circuit chip aredirectly bonded. Such flip chip mounting has the advantage that manybumps can be formed even on the bottom surface of a semiconductorintegrated circuit chip of a limited space.

As a method for connecting bumps and electrodes, attempts have been madeto seal a semiconductor integrated circuit chip and a wiring board by aresin to connect and affix them.

Summarizing the problem to be solved by the invention, a large number ofsemiconductor integrated circuit chips are of a type using silicon fortheir semiconductor substrates. The linear thermal expansion coefficientof a silicon chip is much smaller than that of a wiring board. Forexample, the former is no more than 10% of the latter. As a result ofthe large difference of the linear thermal expansion coefficients,thermal stress appears when the temperature changes.

In most cases, the linear thermal expansion coefficient of thesemiconductor integrated circuit chip is also largely different fromthat of the sealing resin. Similarly, the linear thermal expansioncoefficient of the wiring board is often different from that of thesealing resin.

As a result of the difference of the linear thermal expansioncoefficients, when the temperature rises during operation of thesemiconductor integrated circuit chip, thermal stress appears betweenthe semiconductor integrated circuit chip and the wiring board betweenwhich the sealing resin is interposed.

In a flip-chip structure designed for compactness, there is no mechanismfor easing stress such as a lead frame. Therefore, if flip chip mountingis adopted, there could be deformation of the semiconductor integratedcircuit chip, decline of bonding between bumps and electrodes, or evenloss of the bonded state.

As shown here, although a reduced size is aimed at with flip chipmounting, sometimes the reliability of the semiconductor device coulddecline because of poor bonding or loss of bonding caused by thermalstress.

Accordingly, it is desirable to improve the reliability against thermalstress for a flip chip mounting semiconductor device which does not havea mechanism such as a lead frame for easing stresses.

SUMMARY OF THE INVENTION

An object of the present invention is to find the conditions forincreasing the reliability against thermal stress in a flip chipmounting structure.

Another object of the present invention is to provide a flip chipmounting semiconductor device of high reliability and a method forfabricating such a semiconductor device on the basis of the aboveconditions.

Still another object of the present invention is to provide a method forforming bumps on a semiconductor integrated circuit chip on the basis ofthe above conditions.

According to a first aspect of the present invention, there is provideda semiconductor device comprising a wiring board formed with a pluralityof electrodes, a semiconductor integrated circuit chip formed with aplurality of bumps, and a sealing resin for bonding the electrodes andbumps at corresponding positions and further surrounding the bondingportions of the electrodes and bumps to adhere the wiring board andsemiconductor integrated circuit chip, wherein each individual bump isformed under the condition that quantities ΦA, H, and F are in theregion defined by the following formula A:

a _(L)<((ΦA×F)/H)<a _(u)  (A)

where,

ΦA denotes the top diameter of a bump bonded with an electrode,

H denotes the height of a bump, defined as the distance from thesemiconductor integrated circuit chip to the end of the bump bonded withan electrode,

F denotes the linear thermal expansion coefficient of the sealing resin,

a_(L) denotes the lower limit, and

a_(u) denotes the upper limit.

Up until now, it was thought that high bumps were preferable. This isbecause the large number of bumps formed on a semiconductor integratedcircuit chip are uneven in height, the large number of electrodes formedon the wiring board are uneven in height, the bottom surface of thesemiconductor integrated circuit chip is not completely flat, and thesurface of the wiring board is not completely flat. Furthermore, when abump and a electrode are bonded, the semiconductor integrated circuitchip and the wiring board may warp or deform. They may also deform dueto shrinkage of the sealing resin at time of curing. High bumps arepreferable when considering the margin for eliminating such unevenness.

Studies and experiments of the inventors of the present invention haverevealed that there are optimal values to dimensions of individual bumpsdefined by the above formula A.

Note that when considering the height of bumps, the height of electrodesformed on the wiring board should be considered, too.

This is because due to the height of the bumps and the height of theelectrodes, a gap is maintained between the wiring board and thesemiconductor integrated circuit chip to prevent contact of the wiringboard and the semiconductor integrated circuit chip, and the conditionsof the sealing resin are optimized for interposition between the wiringboard and the semiconductor integrated circuit chip for maintainingbonding of bumps and electrodes and for sealing.

Preferably, the sealing resin is a thermosetting resin.

Further, as a condition for the above formula to stand, the linearthermal expansion coefficient of the wiring board is 10 times that of asemiconductor integrated circuit chip and the linear thermal expansioncoefficient of the thermosetting sealing resin is in the range from 20to 70 ppm.

Specifically, the bump comprises gold, and the electrode comprises aconductive metal.

It is desirable that the electrode be harder than the bump. Whenapplying pressure for bonding, it is desirable that the electrode notdeform while the bump deforms.

Preferably, the lower limit a_(L) is about 100, and the upper limita_(u), is about 125.

Specifically, the bump height H is in the range from about 20 μm toabout 25 μm.

More specifically, the top diameter ΦA of a bump is no more than 50 μm.

These regions of the bump height and top diameter ΦA are realistic bumpdimensions giving good reliability against thermal stress.

According to a second aspect of the present invention, there is provideda semiconductor device comprising a wiring board formed with a pluralityof electrodes, a semiconductor integrated circuit chip formed with aplurality of bumps, and a sealing resin for bonding the electrodes andbumps at corresponding positions and further surrounding the bondingportions of the electrodes and bumps to adhere the wiring board andsemiconductor integrated circuit chip, wherein each individual bump isformed under the condition that quantities ΦA and L are in the regiondefined by the following formula B,

(b ₁ ×L/2)<ΦA<(b ₂ ×L/2)  (B)

where,

ΦA denotes the top diameter of a bump bonded with an electrode,

L denotes the interval (distance) between adjacent bumps,

b₁ denotes a first coefficient, and

b₂ denotes a second coefficient.

Because the shorter the interval between adjacent bumps, the larger thenumber of bumps able to be formed, a short interval is desirable.However, studies and experiments of the inventors of the presentinvention have found there is a limit defined by the above formula B.

For example, the first coefficient b₁ is about 0.75, and the secondcoefficient b₂ is about 0.85.

In addition, according to a third aspect of the present invention, thereis provided a semiconductor device comprising a wiring board formed witha plurality of electrodes, a semiconductor integrated circuit chipformed with a plurality of bumps, and a sealing resin for bonding theelectrodes and bumps at corresponding positions and further surroundingthe bonding portions of the electrodes and bumps to adhere the wiringboard and semiconductor integrated circuit chip, wherein each individualbump is formed under the condition that quantities ΦA, H, and F are inthe region defined by the above formula A, and the interval betweenadjacent bumps is in the region defined by the above formula B.

The semiconductor device according to the third aspect of the presentinvention is a combination of the semiconductor device satisfying theconditions of bump dimensions according to the above first aspect andthe semiconductor device satisfying the conditions of the bump intervalaccording to the above second aspect.

According to the third aspect of the present invention, there is able toprovide a semiconductor device displaying reliability against thermalstress and able to be arranged with a practical number of bumps.

In addition, according to a fourth aspect of the present invention,there is provided a method for forming a plurality of bumps on asemiconductor integrated circuit chip used for a semiconductor devicecomprising a wiring board formed with a plurality of electrodes, asemiconductor integrated circuit chip formed with a plurality of bumps,and a sealing resin for bonding the electrodes and bumps atcorresponding positions and further surrounding the bonding portions ofthe electrodes and bumps to adhere the wiring board and semiconductorintegrated circuit chip, wherein (a) adjacent bumps are formed under thecondition that the interval between adjacent bumps is defined by theabove formula B, and (b) each individual bump before bonding with anelectrode is formed to have an initial height H0 including an additionalpart for compensating for deformation caused by bonding so that thedimensions ΦA and H and the quantity F of each individual bump are inthe region defined by the formula A, and each individual bump is formedin a near spherical shape so that the top diameter ΦA satisfies theformula A after bonding with an electrode,

A bump before bonding is formed to have a larger height to include apart for exactly compensating for deformation caused by a pressureduring bonding.

In addition, according to a fifth aspect of the present invention, thereis provided a method for fabricating a semiconductor device comprising awiring board formed with a plurality of electrodes, a semiconductorintegrated circuit chip formed with a plurality of bumps, and a sealingresin for bonding the electrodes and bumps at corresponding positionsand further surrounding the bonding portions of the electrodes and bumpsto adhere the wiring board and semiconductor integrated circuit chip,the method comprising:

(a) a step for forming bumps, wherein adjacent bumps are formed underthe condition that the interval between adjacent bumps is defined by theabove formula B, and each individual bump before bonding with anelectrode is formed to have an initial height H0 including an additionalpart for compensating for deformation caused by bonding so that thedimensions ΦA and H and the quantity F of each individual bump are inthe region defined by the formula A, and each individual bump is formedin a near spherical shape so that the top diameter ΦA satisfies theformula A after bonding with an electrode,

(b) a step, which is separate from the bump forming step, for formingelectrodes on the wiring board, wherein adjacent electrodes are formedunder the condition that the interval between adjacent electrodes isdefined by the above formula B, and each electrode is formed so that itshead is same as or larger than the top diameter ΦA of a bump,

(c) a step for heating a film-like thermosetting resin on a surface ofthe wiring board formed with electrodes at a first heating temperatureand pressing by a first pressure within a first time period totemporarily fix the thermosetting resin on the wiring board,

(d) a step for putting together the semiconductor integrated circuitchip and the wiring board so that each bump formed on the semiconductorintegrated circuit chip and each electrode formed on the wiring boardface each other with the thermosetting resin interposed between them,and

(e) a step for heating and pressing at a temperature higher than thefirst heating temperature, by a second pressure, and within only asecond time period, to satisfy the formula A while the film-likethermosetting resin is interposed between each bump and each electrodethat face each other, wherein each bump pushes the film-likethermosetting resin apart and is bonded with an electrode at acorresponding position.

Preferably, the second heating temperature is higher than the glasstransition temperature of the thermosetting resin. Because of such aheating temperature, the sealing resin will be completely set in a glassstate to reliably bond bumps and electrodes and firmly adhere the wiringboard and the semiconductor integrated circuit chip to seal them.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clearer from the following description of the preferredembodiments given with reference to the accompanying drawings, in which:

FIG. 1 is a sectional view of the outer appearance of a semiconductoraccording to an embodiment of the present invention;

FIG. 2A to 2C are partial views illustrating a basic method for forminga bump before bonding with an electrode;

FIGS. 3A and 3B are partial views continuing from FIG. 2A to 2C,illustrating a basic method for forming a bump before bonding with anelectrode;

FIG. 4 is an enlarged view of bumps just formed on a semiconductorintegrated circuit chip (IC chip) and electrodes formed on a wiringboard;

FIG. 5 shows the results of inspection of the thermal fatigue lifetimeup to destruction of the semiconductor device in FIG. 1 under atemperature cycle test in a range from −25° C. to 125° C. when forming asemiconductor device by using the resins listed in Table 1 as thesealing resin of the present embodiment;

FIG. 6 is a view illustrating the effect of the difference of linearthermal expansion coefficients of a wiring board, an IC chip, and asealing resin on a semiconductor device when applying a negativetemperature load;

FIG. 7 shows graphs of the results of an analysis of decomposing thestress acting on the bonding surface of a bump with an electrode;

FIGS. 8A to 8C are views of the stress acting on a bump and an electrodewiring pattern when the bump height is large;

FIGS. 9A to 9C are views of the stress acting on a bump and an electrodewiring pattern when the bump height is small; and

FIG. 10 shows graphs of the relation between the bump height and theequivalent-stress when changing the diameter (referred to as topdiameter ΦA) of a bump in contact with an electrode wiring pattern.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, a preferred embodiment according to the present invention will bedescribed with reference to the accompanying drawings.

FIG. 1 is a sectional view of the outer appearance of a semiconductordevice according to an embodiment of the present invention.

The semiconductor device 1 illustrated in FIG. 1 comprises a wiringboard 12 and a silicon IC chip 14 as examples of a wiring board and asemiconductor integrated circuit chip of the present invention.

On the surface of the wiring board 12 that faces the IC chip 14, anumber of electrodes 122 are formed.

When the wiring board 12 and the IC chip 14 are placed togetherface-to-face, the electrodes 122 and bumps 142 at correspondingpositions contact each other, and the wiring board 12 and IC chip 14 areelectrically connected.

Between the wiring board 12 and IC chip 14, there is a predetermined gapover which the electrodes 122 and bumps 142 are connected. In this gap,a thermosetting resin, for example, an epoxy resin or other sealingresin 16 is interposed and is then heated to bond the wiring board 12and the IC chip 14 and seal the surrounding of the electrodes 122 andbumps 142.

Method for Forming Bumps

A basic method for forming bumps before bonding with electrodes isdescribed below with reference to FIG. 2A to 2C and FIGS. 3A and 3B.

In FIG. 2A, an IC chip 14 is placed on a heatable working stand 102.

On the IC chip 14, an aluminum pad 144 is formed in a region where abump will be formed. A passivation film 146 is formed around the pad.

Above the IC chip, a capillary 104 is provided. The capillary 104 isformed with a hole for passing a gold wire 106. Above the capillary 104,a clamper 110 is provided for holding the gold wire 106. In FIGS. 2A to2C, the clamper 106 holds the upper end of the gold wire 106.

The capillary 104 is moved up and down by a not illustrated hoisting andlowering mechanism. In FIG. 2A, the capillary 104 is moved up at ahigher position.

Above the IC chip 14 near the front end of the capillary 104, a torch112 is provided.

In FIG. 2A, a high voltage is applied to the torch 112, causing thetorch 112 to discharge and spark to the end of the gold wire 106positioned at the end of the capillary 104 positioned near the torch112. Due to this, the end of the gold wire 106 sticking out from the endof the capillary 104 turns into a gold small spherical body(hereinafter, referred to as a gold ball) which is the forerunner of abump.

The gold ball 120, which is illustrated as a bump 142 in FIG. 4, isformed in a near spherical shape at the end of the capillary 104.

Below, a brief description will be made of the size of the gold ball120. The dimensions of the bump 142 will be briefly explained first, anddetails will be given later. As described in the fourth aspect of thepresent invention, ΦA, H, and F are used to denote the top diameter of abump bonded with an electrode, the height of a bump defined as thedistance from the semiconductor integrated circuit chip to the end ofthe bump bonded with an electrode, and the linear thermal expansioncoefficient of the sealing resin, respectively. By a leveling operationemploying a leveling plate 114 in FIG. 3B and in the way illustrated inFIG. 4, a bump before bonding with an electrode 122 is formed to have aninitial height H0 including a part that will be deformed during thebonding process so that the dimensions of the bump 142 will be in theregion defined by the above formula A. Namely, it is desirable that abump not bonded with an electrode 122 yet (gold ball 120) is formed tohave a larger height to include exactly the part that will be deformedby the pressure during bonding. In consideration of the deformationcaused by such a leveling operation, it is desirable that the gold ball120 is formed somewhat larger.

The bumps illustrated in FIG. 4 are formed in the above way and thenpressed against aluminum pads 144 by a method described later withreference to FIG. 2B and FIG. 2C, whereby part of each bump is changedin shape. Note that the definition of the up-down direction in FIG. 4 isreverse to that in FIGS. 2A to 2C.

In FIG. 2B, the clamper 110 is opened and the gold wire 106 is released.The capillary 104 is lowered toward the aluminum pad 144 on the IC chip14, and the gold ball 120 formed at the end of the capillary 104 ispressed against the aluminum pad 144 by applying a certain pressure. Inthis state, an ultrasonic wave is applied and the working stand 102 isheated so that the gold ball 120 is melted and fixed on the aluminum pad144.

After the gold ball 120 is fixed on the aluminum pad 144, as shown inFIG. 2C, the capillary 104 is hoisted away from the aluminum pad 144 sothat the gold wire 106 projects a predetermined length (tail length) outof the capillary 104 for forming the next gold ball. During thisoperation, the clamper 110 is opened to keep the gold wire 106 released.

In FIG. 3A, the clamper 110 is closed to hold the gold wire 106. Then,the capillary 104 is moved upward, hence the gold wire 106 is undertension. Due to this tension, at the boundary between the crystalportion of the gold wire 106 and the re-crystalized portion formed bythe spark, the gold ball 120 portion is torn off from the gold wire 106and a bump is formed.

The distance by which the capillary 104 is moved up defines the lengthof the gold wire 106 sticking out of the end of the capillary 104. Thelatter in turn defines the size of the gold ball 120 formed by the sparkfrom the torch 112, so it is the ascent or descent distance of thecapillary 104 that defines the size of the bump 142 before bonding (goldball 120). Consequently, in the present embodiment, the size of the goldball 120 is controlled by the distance moved by the capillary 104.

By this method, a desired number of bumps can be formed sequentially.For this, for example, the working stand 102 may be shifted.

After a desired number of bumps are formed, as shown in FIG. 3B, aleveling plate 114 is placed on these bumps 142 formed on the surface ofthe IC chip 14 to apply a leveling load so that all the bumps 142 formedon the surface of the IC chip 14 have the same specified height.

While details of the dimensions of the bump 142 will be given later,here, as described in the fourth aspect of the present invention, ΦA, H,and F are used to denote the top diameter of a bump bonded with anelectrode, the height of a bump defined as the distance from thesemiconductor integrated circuit chip to the end of the bump bonded withan electrode, and the linear thermal expansion coefficient of thesealing resin, respectively. In order for the dimensions of bumps 142 tobe in the region defined by the above formula A, it is desirable thateach individual bump before bonding with an electrode 122, asillustrated in FIG. 4, is formed to have an initial height H0 includinga part which will be deformed during bonding, and a leveling plate 114is employed to unify the heights of all these bumps. Namely, it isdesirable that a bump not bonded with an electrode 122 yet (gold ball120) is formed to have an excess height to include exactly the part thatwill be deformed by the pressure during bonding.

Consequently, as shown in FIG. 4, a number of bumps 142 each having abottom diameter ΦB and height H0 are formed on the surface of the ICchip 14 at a pitch interval L.

FIG. 4 is an enlarged view of bumps 142 just formed on the IC chip 14and electrodes 122 formed on a later explained wiring board 12.

Method for Forming Electrodes

Below, a description will be given of a method for forming electrodes122 on the wiring board 12.

The electrodes 122 of height H₄ are formed, as shown in FIG. 4, at thesame pitch interval as the bumps 142 and at positions corresponding tothose many bumps on the IC chip 14 using for example aluminum, copper,or another conductive metal by a well known method of formingelectrodes.

In the present embodiments for example, the electrode 122 is formed byusing for example copper as its basic portion, and is nickel plated andgold plated then.

The basic material copper is a conductive metal harder than gold of thebumps 142. While bumps 142 are deformed when they are pressed againstthe electrodes 142 during bonding, it is desirable that the conductivemetal forming the basic portion of the electrodes 122 have a high enoughrigidity that the electrodes 122 do not deform. Copper may also bereplaced by other metals, for example, aluminum.

The outermost gold plating is for raising the bonding ability of bothrelatively soft metals and preventing erosion when bonding with the goldbumps 142.

The intermediate nickel plating is for enabling stable plating of goldon the surface of copper.

Method for Fabricating Semiconductor Device 1

Below, a description will be given of a method for putting together thewiring board 12 formed with an electrode wiring pattern 122 and the ICchip 14 formed with bumps 142 face-to-face so that the electrode wiringpattern 122 and bumps are connected and for sealing with a sealing resinby the method described above to form the semiconductor device 1 shownin FIG. 1.

(a) A film-like bonding layer of the same size as the IC chip 14 isprepared. As the film-like bonding layer, use is made of for example afilm-like bonding layer of a thermosetting adhesive, for example, anepoxy resin.

(b) This film-like bonding layer is placed on the surface of the wiringboard 12 where the electrodes 122 are formed. For temporary fixing, forexample, this layer is heated to the first heating temperature of 80° C.and applied with the first pressure of 3 kg/cm² for the first timeperiod of 3 seconds as defined in the present invention. By this heatingand pressure, the film-like bonding layer is thermally set andtemporarily fixed on the wiring board 12.

(c) Next, the surface of the IC chip 14 where the bumps 142 are formedis made to face the surface of the wiring board 12 where the electrodes122 are formed, and the corresponding electrodes 122 and bumps 142 arepressed against each other with the thermally set film-like bondinglayer in between.

(d) Under this condition, the IC chip 14 and the wiring board are givena higher pressure and heated to a higher temperature than in thepreceding temporary fixing step, for example, heated to the secondheating temperature of 180° C. to 239° C. and given a second pressure of3 to 5 kg/cm² for the second time period of 20 to 30 seconds as definedin the present invention.

By this heating and pressure, the bumps 142 push apart the film-likebonding layer thermally set in a temporary fixing state and are indirect contact (bonding) and electrical connection with electrodes 122at corresponding positions.

The second heating temperature described above of for example 180° C. to239° C. is one example of a temperature higher than the glass transitiontemperature of the thermosetting resin. It is preferable that thisheating temperature is higher than the glass transition temperature ofthe thermosetting resin. Examples of the glass transition temperaturesof the sealing resin 16 are shown in Table 1.

Table 1 presents the linear thermal expansion coefficients, modules ofelasticity, and glass transition points of sealing resin types A to H.

TABLE 1 Types and Material Properties of Sealing Resins Linear Module ofGlass Type of thermal expansion elasticity transition temp- resincoefficient α1 (ppm) (GPa) erature (Tg) A 21 7.0 163 B 23 6.5 140 C 299.2 140 D 32 4.0 138 E 38 3.5 138 F 40 4.9 140 G 47 4.5 119 H 60 2.4 134

In the present invention, designed to avoid the problem of thermalstress, as described later in detail, although the magnitude of thelinear thermal expansion coefficient of a sealing resin 16 is ofimportance, as the sealing resin 16 of the present embodiment, use canbe made of the sealing resins shown in Table 1 which have linear thermalexpansion coefficients (ppm) ranging from 20 to 60 or so or, in a widerregion, from 15 to 70 or so.

The second pressure causes the bumps 142 to push apart the film-likebonding layer thermally set in a temporary fixing state to come intodirect contact with the electrodes 122 at corresponding positions, anddeforms the bumps 142 to a certain degree, but it is not so strong as todeform and damage the electrodes 122 . For example, it is in the rangefrom 3 to 5 kg/cm².

Due to the above process, the semiconductor device 1 of the structureillustrated in FIG. 1 is formed. Namely, in the state where the bumps ofthe IC chip 14 and the corresponding electrodes 122 of the wiring board12 are electrically connected, their surroundings are sealed by thesealing resin 16 and the wiring board 12 and the IC chip 14 are adhered.

FIG. 5 shows the results of inspection of the thermal fatigue lifetimeup to destruction of the semiconductor device in FIG. 1 under atemperature cycle test in a range from −25° C. to 125° C. when forming asemiconductor device 1 by using the resins A to H listed in Table 1 asthe sealing resin 16.

In FIG. 5, the horizontal axis represents the number of temperaturecycles, while the vertical axis represents the equivalent-stress (MPa).Note that the equivalent-stress is the von Misess stress.

The results presented in FIG. 5 show that the magnitude of the linearthermal expansion coefficient of the sealing resin 16 does not directlyinfluence the maximum stress of the sealing resin 16, while the maximumstress acting on. the gold bump 142 increases with a rise of the linearthermal expansion coefficient of the sealing resin 16, and the lifetimeof the bump 142 becomes short when the linear thermal expansioncoefficient of the sealing resin 16 is large. This is because if thelinear thermal expansion coefficient of the sealing resin 16 is high,the sealing resin 16 displaces due to thermal stress, and because of thedisplacement, the bonding portions of the bumps 142 and electrodes 122are warped. Further, if repeatedly warped, the bonding between bumps 142and electrodes 122 turns poor or may even be loosened.

From the viewpoint of the thermal stress, a small linear thermalexpansion coefficient of the sealing resin 16 results in highreliability.

If heating in the above way, as illustrated in FIG. 6, because of thedifference of linear thermal expansion coefficients of the wiring board12, IC chip 14, and sealing resin 16, thermal stress begins to act onthe semiconductor device 1.

FIG. 6 is a view of a state when a negative temperature load is applied.

The linear thermal expansion coefficient of the IC chip 14 comprisingmainly silicon is 10 times smaller than that of the wiring board 12, sowhen a negative temperature load is applied, the contraction of thewiring board is larger than the IC chip 14, and the wiring board bendsto the side of the IC chip 14.

If the semiconductor device 1 is heated to the high temperature side,because the linear thermal expansion coefficient of the sealing resin 16is larger than that of the electrode 122 side of the wiring board 12,the electrode 122 side of the wiring board 12 expands while the oppositeside contracts. In the same way, because the linear thermal expansioncoefficient of the bump side 142 of the IC chip 14, which is mainlycomprised of silicon, is smaller than that of the sealing resin 16, thesealing resin side of the IC chip 14 expands and the opposite sidecontracts.

On the contrary, if the semiconductor device 1 is cooled to the lowtemperature side, expansion and contraction will occur in a way contraryto that shown in FIG. 8.

Such expansion and contraction cause thermal stress acting on the goldbumps 142 of the IC chip 14. FIG. 5 shows the results associated withthis situation.

From the results of FIG. 5, it is clear that the thermal fatiguelifetime of the semiconductor device is dominated by the amplitude ofthe stress occurring on the bonding surface between the gold bumps 142formed on the IC chip 14 and the wiring pattern 122 formed on the wiringboard 12.

Accordingly, in order to improve the reliability of the connection ofthe semiconductor device 1 against thermal stress, it is desirable toreduce as small as possible the amplitude of the stress which occurs onthe bonding surface between the bumps 142 and the electrode wiringpattern 122.

Therefore, the inventors of the present invention did an analysis ofdecomposing the stress acting on the bonding surface of a bump 142 thatis in contact with an electrode 122. The results are presented in FIG.7. In FIG. 7, the horizontal axis is the bump height, and the verticalaxis is the amplitude of the equivalent-stress.

Note that the results shown in this figure were obtained under theconditions of a size of the IC chip 14 of 9 mm×0.4 mm, a thickness ofthe wiring board 12 of 0.7 mm, a bump 142 of a diameter of 30 μm at itsfront end and of a contact diameter of 23 μm with the electrode wiringpattern 122 , and a core material including a 50 μm build-up layer ofFR-4.

The central curve shows the amplitude of the equivalent-stress which wasobtained by numerical calculation such as the limited element method.

σ_(z) represents the amplitude of stress in Z direction, namely, alongthe Z—Z axis in FIG. 1.

τ_(xz) stands for the amplitude of a shearing stress relevant to the Z—Zdirection and the horizontal direction (X direction) perpendicular tothe Z—Z direction.

The curve of the amplitude of the equivalent-stress shows theequivalent-stress is a minimum when the bump height is about 18 μm. Thereason is examined below with reference to FIGS. 8A to 8C.

FIGS. 8A to 8C are views illustrating the stress acting on the bumps 142and electrodes 122.

A larger height of bumps 142 implies a thicker sealing resin 16. Thatis, the sealing resin 16 contributes to the compressing force orcontracting force.

For example, when the sealing resin 16 around the cooled bumps 124contracts, because the total contraction of the sealing resin 16 in itsthickness direction is proportional to its thickness, it increases withthe thickness of the sealing resin 16. If this contraction evenlydistributes over the entire bump 142, the contraction per unit length(per unit length along the thickness direction) is the same, so thestress on the bump 142 does not increase. This phenomenon happens mainlynear the bonding surface between the bump 142 and the electrode wiringpattern 122 , which receives a contracting force from the sealing resin16 and makes the bump 142 contract. Therefore when the bump 142 is high,namely, as illustrated in FIG. 8C, the thickness of the sealing resin 16is higher than that in FIG. 8B. As the stress σ_(z) acting on the bump142 increases, the above phenomenon occurs.

In FIG. 8C, warping occurs locally mainly along the edge of the bondingsurface of a bump 142 and an electrode 122.

Next, the case of a lower bump 142 will be considered with reference toFIGS. 9A to 9C.

As shown in FIG. 9B, a lower bump 142 implies a close distance betweenthe IC chip 14 having a small linear thermal expansion coefficient andthe wiring board 12 having a large linear thermal expansion coefficient.As a result, a shearing stress acts on the bump 142.

If the bump 142 is low, because the metal structures of the IC chip 14and the wiring board 12 are close, the semiconductor device 1 is bendedmore, and the shearing stress τ_(xz) per unit volume increases.

If the bump 142 is high, as shown in FIG. 9C, contrary to FIG. 9B, thewarping of the device 1 becomes smaller.

As illustrated in FIG. 5, there is a strong correlation between thethermal fatigue lifetime and the equivalent-stress occurring on the bump142, so as shown in FIG. 7, the fact that the equivalent-stress is at aminimum relative to the height of the bump 142 implies there is anoptimum height of the bump 142 that optimizes the reliability of thesemiconductor device 1 against thermal stress.

The inventors of the present invention performed experiments to searchfor such an optimum value. The results are presented in FIG. 10,

FIG. 10 shows graphs of the relation between the bump height and theequivalent-stress when changing the diameter (referred to as a topdiameter ΦA) of a bump 142 in contact with an electrode 122 to ΦA=30 μm,40 μm, and 50 μm. The horizontal axis represents the bump height, andthe vertical axis represents the equivalent-stress.

FIG. 10 shows that there is an optimum value of the bump height thatgives a minimum amplitude of the equivalent-stress for a constant topdiameter ΦA. Further, the minimum of the amplitude of theequivalent-stress increases with a decreasing top diameter ΦA of thebump.

Examining the graph of FIG. 10 in more detail, as shown by the twodashed lines, there are an optimum height H and an optimum top diameterΦA of a bump that give a constant ratio ΦA/H with respect to the bumpheight H.

Further, from the fact that the optimum height of a bump 142 increaseswith a decreasing linear thermal expansion coefficient F (ppm) of thesealing resin 16, the dimensions of a bump become optimum in the regionfrom a_(L) to a_(u), which is the region satisfying the followingformula (1):

a _(L)<((ΦA×F)/H)<a _(u)  (1)

As shown in FIG. 4 and FIG. 9C, ΦB, H0, H, and ΦA are used to denote thebottom diameter of the bump 142, the initial height of the bump 142, theheight of the bump 124 in contact with an electrode 122 , and the topdiameter, that is, the diameter of the contacting portion of the bump142 bonded with an electrode 122.

The height of an electrode is represented by Hd.

After studying various situations of the top diameter ΦA from theresults in FIG. 10, it is found ΦA=45 μm and H=20 μm are the optimumvalues when F=50 ppm.

After experiments on various linear thermal expansion coefficients F ofthe sealing resin 16 presented in Table 1 and various top diameters ΦA,it is found for a gold bump 142, the preferable height H and topdiameter ΦA of the bump 142 satisfy the following formula. Namely, whena_(L)=100 and a_(u)=125 in formula (1),

100<((ΦA×F)/H)<125  (2)

In addition, if considering the matter from a different point of view, alarger top diameter ΦA of a bump can decrease the amplitude of theequivalent-stress and thus raise the reliability of the semiconductordevice 1 against thermal stress. However, because of the constraintsfrom the interval with adjacent bumps 142 and electrodes 122 , the topdiameter ΦA of a bump 142 can not be made unlimitedly large.

As shown in FIG. 4, if representing the interval between adjacent bumpsas L (μm), it is found the region defined by the following formula (3)is the permitted region in which the IC chip 14 and the wiring board 12can be bonded.

(b ₁ ×L/2)<ΦA<(b ₂ ×L/2)  (3)

According to the experiments by the inventors, if the coefficientsb₁=0.75 and b₂=0.85, a flip chip of high reliability against thermalstress and a long lifetime can be realized.

(0.75×L/2)<ΦA<(0.85×L/2)  (4)

Therefore, in principle, what has to be done is just to form bumpssatisfying the formulae (1) and (3) (or formulae (2) and (4)).

In practice, however, if the bump height is less than 20 μm, because thewiring board 12 sinks when the bumps 142 and the electrode wiringpattern 122 are pressed for bonding, the wiring board 12 may touch theIC chip 14, and there may be insufficient bonding among the many bondedbumps 142 and the electrode wiring pattern 122.

From this point of view, it is concluded the bump height cannot be lessthan 20 μm.

Further, it is also clarified if the top diameter ΦA is less than 50 μm,the optimum bump height H will not be 25 μm or more as long as thelinear thermal expansion coefficient of the sealing resin 16 is not toosmall.

From this point of view, when the top diameter ΦA is less than 50 μm,the bump height is simply defined by the following formula (5):

20<H(μm)<25  (5)

Note that strictly the height of a bump is determined by both theinitial height H0 of the bump and the height Hd of an electrode 122. Inthe present embodiment, the bump height H was studied assuming the totalheight Hd of an electrode 122 is Hd=16 μm including for example 12 μmthick copper, 3 to 5 μm thick nickel plating, and 0.02 μm thick goldplating.

Therefore, when the height Hd of an electrode 122 is smaller than theabove value, the bump height can be made larger accordingly.

In the above embodiment, the electrode 122 was assumed to be comprisedof 12 μm thick copper covered by 3 to 5 μm thick nickel plating and 0.02μm thick gold plating, gold was used for the bump 142, and the sealingresins 16 listed in Table 1 were used, but the present invention is notlimited to this embodiment. Numerous materials and thicknesses areapplicable.

In addition, as an example, the semiconductor integrated circuit chip ofthe present invention is described as an IC chip using silicon, but theinvention is not limited to silicon-based IC chips. For example, a chipusing a compound semiconductor may also be used.

Numerous modifications can also be made without departing from the basicconcept and scope of the present invention.

Summarizing the effects of the invention, according to the presentinvention, the conditions have been found for avoiding poor bonding orlosing bonding between a projecting electrode (bump) formed on asemiconductor integrated circuit chip and an electrode formed on awiring board even under thermal stress.

According to the present invention, based on such conditions, a flipchip semiconductor device of high reliability can be provided.

What is claimed is:
 1. A semiconductor device comprising a wiring boardformed with a plurality of electrodes, a semiconductor integratedcircuit chip formed with a plurality of bumps, and a sealing resin forbonding the electrodes and bumps at corresponding positions and furthersurrounding the bonding portions of the electrodes and bumps to adheresaid wiring board and semiconductor integrated circuit chip, whereineach individual bump and adjacent bumps are formed under the conditionthat quantities ΦA and L are in the region defined by the followingformula B, (b ₁ ×L/2)<ΦA<(b ₂ ×L/2)  (B) where, ΦA denotes a lopdiameter of a bump bonded with an electrode; L denotes an interval(distance) between adjacent bumps; b₁ denotes a first coefficient; andb₂ denotes a second coefficient; wherein said first coefficient b₁ isabout 0.75, and said second coefficient b₂ is about 0.85.